This invention relates to viscosity index improvers (VII's) utilized in producing a dispersant-antioxidant olefin copolymer (DAOCP), and more particularly to a method of retarding the darkening of a DAOCP.
In producing DAOCP(s) VI improvers, a reactive polymer intermediate is used such as grafted maleic anhydride on an olefin copolymer (OCP) and involves derivatizing the polymer bound maleic anhydride further with an excess of an analine or amine bearing hindered amine antioxidant such as N-phenyl phenylene diamine. This entire manufacturing process is generally conducted under an inert nitrogen atmosphere. The product is then stored at 170.degree. F. under a nitrogen atmosphere.
A problem sometimes encountered with this DAOCP VI improver is color darkening when stored at elevated temperatures in the presence of air or oxygen. This is cosmetically unacceptable to some customers though it is not deleterious to it's performance. This darkening is due to oxidation and dimerization of the unreacted excess amine, as in the case of N-Phenyl phenylenediamine, through the primary amine functionality to form a deep blackish blue highly chromophoric material.
Thus, it is an object of the present invention to provide a means of retarding the darkening of a finished DAOCP VI Improver (VII).